Detection and classification of gamma ray emitters has attained heightened importance in the protection of vulnerable targets and populaces from special nuclear materials. Many fissionable special nuclear materials emit gamma rays, due to radioactive decay of the elements therein. However, many less harmful and non-fissionable materials also emit gamma rays. Therefore, it is desirable to be able to identify, and whenever possible, distinguish between the types of gamma ray emitters in an unknown material, possibly further concealed inside of a container or vehicle of some type, such as a car, van, cargo container, etc.
Many types of materials emit gamma rays that appear very close together in a gamma spectrum. Scintillator detectors use materials that emit bursts of light when gamma rays interact with the atoms in the scintillator material. The amount of light emitted in a given scintillation pulse can be used to identify the isotope that is emitting the gamma rays. Scintillator detectors may also be used to detect other types of radiation, such as alpha, beta, neutron and x-rays.
Detection sensitivity for weak gamma ray sources and rapid unambiguous isotope identification is principally dependent on energy resolution, and is also enhanced by high effective atomic number of the detector material. High energy resolution scintillator detectors are useful for resolving closely spaced gamma ray lines in order to distinguish between different gamma-emitting radioisotopes. Generally, gamma ray detectors are characterized by their energy resolution. Resolution can be stated in absolute or relative terms. For consistency, all resolution terms are stated in relative terms herein. A common way of expressing detector resolution is with Full Width at Half Maximum (FWHM). This equates to the width of the gamma ray peak on a spectral graph at half of the highest point on the peak distribution.
Plastic scintillator detectors are widely used for scintillation counting, but due to their low efficiency of photoelectric absorption, are rarely used for gamma ray spectroscopy. Unfortunately plastic scintillators also have low light yields, and more importantly, their low effective atomic number, or “Zeff” results in poor photopeak efficiency, and they therefore have not been used for gamma-ray spectroscopy.
Without wishing to be bound by any theory, it is believed that the foregoing deficiency is due to the following. When excitons are created from high energy radiation, the high energy radiation travels through the plastic, creating a cascade of excitations, which in turn produces excitons in which there is a distribution of triplet and singlet excitons. The ratio at which these excitons are produced is material-dependent, and it is believed that the ratio is about 3 to 1, meaning there are three times more triplets than singlets being formed, which severely diminishes the luminescence light yield of the material due to the fact that standard plastic scintillators contain an emitting dye, or fluor, which is a singlet emitter. The fluor collects singlet excitons and re-emits them, generally disregarding the triplets, thus losing the majority of the excitation. Therefore, the addition of a high Z component, has in the past, reduced the scintillation light yield of the plastic due to the fact that singlet excitons in the material are converted into triplets, via spin-orbit coupling to the high Z component, and since these triplet excitons cannot be collected by the singlet emitter, thus reducing the light yield in proportion to the concentration of high-Z constituent.
Attempts to increase this effective Z by doping with heavy metals to induce a photopeak have been made in the past. It was believed that if it were possible to add a sufficient amount of high Z to a plastic, that one would consequently be able to achieve gamma ray spectroscopy which is normally only possible if there is a high photoelectric cross section of the material that produces a very specific peak, the photo peak that appears in materials with that property. The photo electric cross section scales as Zeff4. Unfortunately, these high Z additives also have a greatly undesirable property in organic photochemistry.
Particularly, studies of loading plastic scintillators with high atomic number organometallics in the past have encountered light yield reductions and yellowing or browning of the scintillator material upon addition of sufficient organometallic to provide photoelectric absorption enhancement.
Accordingly, it is presently widely believed that it is impossible to create a plastic scintillator that provides sufficient resolution for gamma ray spectroscopy.